TERRESTRIAL MAGNETISM IS DEEP IN THE EARTH. 103 



zontal force at a great elevation. But the last set 

 of observations casts much doubt on this conclusion. 

 It is to be remarked that all the balloon-observations at 

 great height were compared with observations on the 

 earth. It might have been safer to compare them with 

 balloon-observations at small elevations. Now the last 

 set of observations seems to shew that an apparent 

 large diminution arises simply from the effect of locali- 

 zation in the balloon-car ; and, if this be accepted, there 

 is scarcely any sensible effect to be ascribed to the 

 great elevations. 



Now, remarking how rapidly magnetic power di- 

 minishes with increase of proportion of distance from 

 the magnetic poles, it follows from the observations 

 above that the height of three or four miles must bear 

 a small proportion to the distance of the magnet which 

 produces the magnetic power observed at the earth's 

 surface, and therefore the source of magnetism must 

 be deep. 



45. Attempt to explain Terrestrial Magnetism by 

 the action of a magnet of small dimensions but of very 

 great power, near the center of the earth. 



About the middle of the last century it was sug- 

 gested by Mayer, and in the present century the same 

 idea was independently adopted by Humboldt and Biot 

 (Biot, Traiti de Physique, 181 6r vol. iii. page 139), that 

 the principal phenomena of Terrestrial Magnetism 

 could be explained by the action of a powerful magnet, 

 of limited dimensions, near the center of the earth. Its 



